A fuel cell stack is a power generation system in which a fuel is oxidized by an electrochemical process, whereby energy emitted through an oxidation reaction is directly converted into electric energy. The fuel cell stack has a membrane-electrode assembly in which a polymer electrolyte membrane for selectively transporting hydrogen ions is sandwiched, on both of its sides, by a pair of electrodes formed of a porous material. Each of the electrodes of the pair contains, as its main constituent, carbon powder that carries a platinum-based metal catalyst, and includes a catalyst layer in contact with the polymer electrolyte membrane and a gas diffusion layer which is formed on the surface of the catalyst layer and which has both permeability and electronic conductivity.
WO2002/015316 discloses a fuel cell vehicle on which a fuel cell system is mounted as a power source. A power storage device mounted on this fuel cell vehicle supplies current to a load when a current supplied from a fuel cell stack is smaller than a current required by the load, while storing regenerative power collected by the load and power generated by the fuel cell stack. The above publication also discloses that the distribution state of the current flowing to the fuel cell stack and the power storage device is controlled suitably, thereby suitably performing charging of the power storage device without increasing the capacity of the power storage device.
Meanwhile, when an output voltage of the fuel cell stack changes to be within the range of high potential, this might advance deterioration of the fuel cell stack. This is because the platinum catalyst contained in the catalyst layer of the membrane-electrode assembly is ionized to dissolve in a high-potential environment. In order to prevent such disadvantages, as disclosed in Japanese laid-open patent publication 2007-109569, a method of performing power generation control so as to prevent an output voltage of a fuel cell stack from exceeding a predetermined upper limit voltage (hereinafter, referred to as high-potential avoidance voltage) (hereinafter, referred to as high-potential avoidance control) has been known.
In such a fuel cell system having a high-potential avoidance control function, power generation of the fuel cell stack occurs through the high-potential avoidance control even when the fuel cell stack is required to generate electric power, and therefore, electric power is provided only to a power storage device in the case of a low load such as an idle stop. In light of such circumstances, when an amount of charge of a power storage device exceeds a predetermined threshold, a high-potential avoidance control has been prohibited, and the voltage of a fuel cell stack has been increased to an open end voltage, thereby avoiding overcharge of the power storage device.    Patent Document 1: WO2002/015316    Patent Document 2: Japanese laid-open patent publication No. 2007-109569